Cement compositions may be used in a variety of subterranean operations. For example, in subterranean well construction, a pipe string (e.g., casing, liners, expandable tubulars, etc.) may be run into a well bore and cemented in place. The process of cementing the pipe string in place is commonly referred to as “primary cementing.” In a typical primary cementing method, a cement composition may be pumped into an annulus between the walls of the well bore and the exterior surface of the pipe string disposed therein. The cement composition may set in the annular space, thereby forming an annular sheath of hardened, substantially impermeable cement (i.e., a cement sheath) that may support and position the pipe string in the well bore and may bond the exterior surface of the pipe string to the subterranean formation. Among other things, the cement sheath surrounding the pipe string functions to prevent the migration of fluids in the annulus, as well as protecting the pipe string from corrosion. Cement compositions also may be used in remedial cementing methods, for example, to seal cracks or holes in pipe strings or cement sheaths, to seal highly permeable formation zones or fractures, to place a cement plug, and the like.
One problem that may be encountered during the placement of a cement composition in a well bore is unwanted gas migration from the subterranean formation into and through the cement composition. Gas migration may be caused by the behavior of the cement composition during a transition phase in which the cement slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. When first placed in the annulus, the cement composition acts as a true liquid and thus transmits hydrostatic pressure. However, during the transition phase, certain events occur that cause the cement composition to lose its ability to transmit hydrostatic pressure. One of those events is the loss of fluid from the slurry to the subterranean formation. Another event is the development of static gel strength in the slurry. As a result, the pressure exerted on the formation by the cement composition may fall below the pressure of the gas in the formation such that the gas may begin to migrate into and through the cement composition. When gas migration begins, the cement composition typically has a gel strength of about 100 lbf/100 ft2. The gas migration may cause flow channels to form in the cement composition. With time, the gel strength of the cement composition increases to a value sufficient to resist the pressure exerted by the gas in the formation against the composition. At this point, the cement composition typically has a gel strength of about 500 lbf/100 ft2. The cement slurry then sets into a solid mass.
Unfortunately, the flow channels formed in the cement during such gas migration remain in the cement composition once it has set. Those flow channels can permit further migration of gas through the set cement composition. Thus, the set cement composition residing in the annulus may be ineffective at maintaining the isolation of the adjacent subterranean formation. To overcome this problem, attempts have been made to design a cement composition having a shorter transition time, i.e., the period of time during which gas migration into the slurry can occur, which is typically the time ranging from when the gel strength of the slurry is about 100 lbf/100 ft2 to when it is about 500 lbf/100 ft2, as measured using a Multiple Analysis Cement System (MACS® II, available from Fann Instrument Company) in accordance with the procedure for determining cement transition times set forth in API RP 10B-6, Recommended Practice on Determining the Static Gel Strength of Cement Formulations, dated Aug. 1, 2010. Gas migration control additives have been developed to provide shorter transition times. One particular additive for controlling gas migration is a copolymer of sodium 2-acrylamido-2-methylpropanesulfonate and N,N-dimethylacrylamide. While this additive can be used to control gas migration, the highest percent activity it can be effectively used as an aqueous solution is 9% by weight above which the solution becomes too viscous for the liquid additive pumps to handle. Other additives that may be used may either be too expensive or may provide transition times that may be longer than desired.